 reserve L for Quasi-Boolean_Algebra,
         x, y, z for Element of L;
 reserve L for Nelson_Algebra,
         a, b, c, d, x, y, z for Element of L;

theorem Th20: :: (2.20)
  a => !b = b => !a
  proof
    (a => !b) = a => (b => (-Top L)) by Def14
      .= b => (a => (-Top L)) by Th18
      .= b => !a by Def14;
    hence thesis;
  end;
